The present invention relates to the detection of aerosols, such as smoke and other combustion aerosols, and more particularly to an improved device and method for detecting such aerosols.
Aerosol detection devices are commonly employed in fire alarm systems to energize alarms therein upon the detection of smoke in the surrounding atmosphere. As a consequence, these devices are frequently referred to as smoke detectors.
Heretofore, smoke detectors have been devised which utilize ionization effects to generate electrical signals to trigger fire alarms. Generally, these devices detect changes in ion current caused by the presence of smoke in the atmosphere being monitored and operate on the hereinafter described principle. The level of an ion current flowing between electrodes of an ion chamber under clean or clear air conditions will decrease upon the introduction of smoke into the region between the electrodes due to capture of ions by the relatively massive and slow moving aerosol particles making up the smoke. Typically, the mass of each aerosol particle is several thousand times larger than the mass of the ions they capture, while the velocities of the aerosol particles are negligible in comparison to the ion current velocity.
Thus, the velocities of the ions once captured by the aerosol particles are reduced substantially to a standstill relative to the ion current, and hence in essence are removed therefrom. The actual drop in ion current is determined by and directly proportional to the number of such ion captures taking place per unit of time.
Prior art smoke detectors of the type utilizing a single ion chamber, i.e., having a single region of ionization, have been constructed. These devices are generally subject to one or more of the below described drawbacks. In order to generate a usable and measurable output signal representative of changes in ion current, a high value resistor, such as 10.sup.11 ohms, commonly was used in conjunction with the ion chamber to form therewith a voltage divider. Many problems are encountered with the use of such high value resistors including stability, handling, high cost, and problems caused by environmental conditions like temperature and humidity. Further, such a single ion chamber has the inherent disadvantage of having no intrinsic temperature, pressure or humidity compensation ability. Certain types of circuitry may be designed to minimize these atmospheric effects on a single ion chamber, however, there is then the added cost of such circuitry.
Prior art smoke detectors of the dual chamber type, also have been devised.
In such devices, two ion chambers are provided with an ion source for each. One of the ion chambers is a reference chamber which must be isolated from smoke in the surrounding atmosphere, while the other chamber is employed as a sensing chamber to sense the smoke in the surrounding atmosphere. A first disadvantage of this type of smoke detector is that two ion sources must be provided. These ion sources which are generally radioactive isotopes, such as alpha particle emitters, are costly. Secondly, in order for such smoke detectors to operate properly under varying temperature, pressure and humidity conditions, their two ion sources must be matched and both ion chambers must be subject to atmospheric changes. The latter requirement poses a design problem in the construction of the reference chamber since smoke must be prevented from entering this chamber, while other atmospheric changes must be allowed to influence it. Further, one of the chambers should be electrically isolated from the remainder of the smoke detector circuitry. This fact limits the physical design of the smoke detector.
Patents of general interest known to applicant are listed below.
______________________________________ Country Patent no. ______________________________________ U.S.A. 2,397,075 U.S.A. 2,440,167 U.S.A. 2,736,816 U.S.A. 2,874,304 U.S.A. 3,018,376 U.S.A. 3,162,846 U.S.A. 3,353,170 U.S.A. 3,500,368 U.S.A. 3,514,603 U.S.A. 3,521,263 U.S.A. 3,560,737 U.S.A. 3,676,678 U.S.A. 3,693,009 U.S.A. 3,710,365 West Germany 1,062,957 ______________________________________
Of particular interest, as illustrative of a variant of the earlier described prior art devices, is the above-listed U.S. Pat. No. 3,693,009 to Sasaki. The ionization smoke detecting device disclosed therein has a single ionization chamber defined between a pair of electrodes with a grid electrode positioned therebetween. A radioactive source is positioned adjacent to one of the outer electrodes of the ion chamber and a linear voltage gradient is established between the outer electrodes under a smoke free condition.
Upon entry of smoke into the ion chamber, the voltage gradient between the outer electrodes becomes non-linear and the grid electrode, which acts as a voltage probe, is appropriately positioned to sense the greatest change of shift in this voltage gradient. As will become apparent from the description of the present invention hereinafter, the Sasaki device and its operation, while of interest as illustrative of the state of the prior art, is distinguishable and embodies a quite different concept from that of the present invention.